Good job! Before we move on to the final quiz for this course, let's review what we've learned:

1. To create a report with history-to-date category revenue values:

   SELECT 
     CategoryName,
     SUM(Amount) AS TotalRevenue 
   FROM OrderItems OI
   JOIN Products P
     ON OI.ProductId = P.ProductId
   JOIN Categories C
     ON P.CategoryId = C.CategoryId
   GROUP BY CategoryName;
   

2. To show category revenue values and their relation to the total revenue, use:

   WITH AllCategories AS (
     SELECT SUM(Amount) AS TotalSum
     FROM Orders
   )
   SELECT 
     CategoryName,
     SUM(OI.Amount) AS CategoryRevenue,
     SUM(OI.Amount) / CAST(AC.TotalSum AS DECIMAL(10,2)) AS TotalRevenueRatio
   FROM AllCategories AC, OrderItems OI
   JOIN Orders O
     ON O.OrderId = OI.OrderId
   JOIN Products P
     ON OI.ProductId = P.ProductId
   JOIN Categories C
     ON P.CategoryId = C.CategoryId
   GROUP BY CategoryName,
     AC.TotalSum;
   

3. To show the category revenue for selected categories for each time period or category revenue for each category and selected time periods, use SUM(CASE WHEN) along with GROUP BY. For instance:

   SELECT 
     DATEPART(Year, OrderDate) AS RevenueYear,
     SUM(OI.Amount) AS TotalRevenue, 
     SUM(CASE WHEN C.CategoryName='Beverages' THEN OI.Amount ELSE 0.0 END) AS BeveragesRevenue,
     SUM(CASE WHEN C.CategoryName='Seafood' THEN OI.Amount ELSE 0.0 END) AS SeafoodRevenue 
   FROM OrderItems OI 
   JOIN Orders O
     ON OI.OrderId = O.OrderId
   JOIN Products P
     ON P.ProductId = OI.ProductId
   JOIN Categories C
     ON C.CategoryId = P.CategoryId
   GROUP BY DATEPART(Year, OrderDate)
   ORDER BY DATEPART(Year, OrderDate);
   

How about a short quiz to finish?